Anaphylaxis is an acutesystemic (multi-system) and severe Type I Hypersensitivity allergic reaction. The term comes from the Greek words ana (against) and phylaxis (protection).[1] Anaphylaxis occurs when a person is exposed to a trigger substance, called an allergen, to which they have already become sensitized. Minute amounts of allergens may cause a life-threatening anaphylactic reaction. Anaphylaxis may occur after ingestion, inhalation (though this is rare), skin contact, or injection of an allergen.[2] The most severe type of anaphylaxis—anaphylactic shock—will usually lead to death in a matter of minutes if left untreated.

An estimated 1.24% to 16.8% of the population of the United States may suffer from anaphylactic reactions, 0.002% of whom may experience fatal results.[3] Most common presentation is sudden cardiovascular collapse (88% of reported cases of severe anaphylaxis).[How to reference and link to summary or text]

Researchers typically distinguish between "true anaphylaxis" and "pseudo-anaphylaxis." The symptoms, treatment, and risk of death are identical, but "true" anaphylaxis is always caused directly by degranulation of mast cells or basophils that is mediated by immunoglobulin E (IgE), and pseudo-anaphylaxis occurs due to all other causes. The distinction is only important for researchers who are studying mechanisms of allergic reactions, and it may frustrate patients who feel they are being told that a life-threatening allergic reaction wasn't "real."

The time between ingestion of the allergen and anaphylaxis symptoms can vary for some patients depending on the amount of allergen consumed and their reaction time. Symptoms can appear immediately, or can be delayed by half an hour to several hours after ingestion. [5] However, symptoms of anaphylaxis usually appear very quickly once they do begin.

Anaphylaxis is a life-threatening medical emergency because of rapid constriction of the airway, often within minutes of onset, which can lead to respiratory failure and respiratory arrest. Brain and organ damage rapidly occurs if the patient cannot breathe. Due to the severe nature of the emergency, patients experiencing or about to experience anaphylaxis require the help of advanced medical personnel. First aid measures for anaphylaxis include rescue breathing (part of CPR). Rescue breathing may be hindered by the constricted airways, but if the victim stops breathing on his or her own, it is the only way to get oxygen to him or her until professional help is available.

Another treatment for anaphylaxis is administration of epinephrine (adrenaline). Epinephrine prevents worsening of the airway constriction, stimulates the heart to continue beating, and may be life-saving. Epinephrine acts on Beta-2 adrenergic receptors in the lung as a powerful bronchodilator (i.e. it opens the airways), relieving allergic or histamine-induced acute asthmatic attack or anaphylaxis. If the patient has previously been diagnosed with anaphylaxis, they may be carrying an EpiPen (or TWINJECT TM) for immediate administration of epinephrine. However, use of an EpiPen or similar device only provides temporary and limited relief of symptoms.

Tachycardia (rapid heartbeat) results from stimulation of Beta-1 adrenergic receptors of the heart increasing contractility (positive inotropic effect) and frequency (chronotropic effect) and thus cardiac output.[6] Repetitive administration of epinephrine can cause tachycardia and occasionally ventricular tachycardia with heart rates potentially reaching 240 beats per minute, which itself can be fatal. Extra doses of epinephrine can sometimes cause cardiac arrest. This is why some protocols advise intramuscular injection of only 0.3–0.5mL of a 1:1,000 dilution.

Some patients with severe allergies routinely carry preloaded syringes containing epinephrine, diphenhydramine (Benadryl), and dexamethasone (Decadron)whenever they go to an unknown or uncontrolled environment.These three injections, taken at the beginning of anaphylaxis, can often bring it under control and avoid a trip to the Emergency Room.

Paramedic treatment in the field includes administration of epinephrine IM (or IV infusion in severe cases), Benadryl IM, steroids such as Decadron, IV Fluid administration and in severe cases, pressor agents (which cause the heart to increase its contraction strength) such as Dopamine for hypotension, administration of oxygen, and intubation during transport to advanced medical care.

In severe situations with profuse laryngeal edema (swelling of the airway), cricothyrotomy or tracheotomy may be required to maintain oxygenation. In these procedures, an incision is made through the anterior portion of the neck, over the cricoid membrane, and an endotracheal tube is inserted to allow mechanical ventilation of the victim.

The clinical treatment of anaphylaxis by a doctor and in the hospital setting aims to treat the cellular hypersensitivity reaction as well as the symptoms. Antihistamine drugs such as Benadryl (which inhibit the effects of histamine at histamine receptors) are continued but are usually not sufficient in anaphylaxis, and high doses of intravenous corticosteroids such as Decadron or Solu-Medrol are often required. Hypotension is treated with intravenous fluids and sometimes vasopressor drugs. For bronchospasm, bronchodilator drugs (e.g. Salbutamol, known as Albuterol in the United States) are used. In severe cases, immediate treatment with epinephrine can be lifesaving. Supportive care with mechanical ventilation may be required.

It is also possible to undergo a second reaction prior to medical attention or using an epipen. It is suggested to seek one to two days of medical care.

The possibility of biphasic reactions (recurrence of anaphylaxis) requires that patients be monitored for four hours after being transported to medical care for anaphylaxis.[7]

The Asthma and Allergy Foundation of America advises patients prone to anaphylaxis to have an "allergy action plan" on file at school, home, or in their office to aid others in case of an anaphylactic emergency, and provides a free
"plan" form anyone can print. Action plans are considered essential to quality emergency care. Many authorities advocate immunotherapy to prevent future episodes of anaphylaxis."allergy fact sheet" Immunotherapy with Hymenoptera venoms is especially effective and widely used throughout the world and is accepted as an effective treatment for most patients with allergy to bees, wasps, hornets, yellow jackets, white faced hornets, and fire ants. "WHO guidelines"

Beta-blockers may aggravate anaphylactic reactions and interfere with treatment.

The greatest success with prevention of anaphylaxis has been the use of allergy injections to prevent recurrence of sting allergy. The risk to an individual from a particular species of insect depends on complex interactions between likelihood of human contact, insect aggression, efficiency of the venom delivery apparatus, and venom allergenicity. According to most authorities, venom immunotherapy has been demonstrated to reduce the risk of systemic reactions below 1% to 3%. One simple method of venom extraction has been electrical stimulation to obtain venom, instead of dissecting the venom sac. An allergist will then provide venom immunotherapy which is highly efficacious in preventing future episodes of anaphylaxis.